1. Field of the Invention
The present invention relates to a semiconductor device using an AlGaInN-based material and a method of fabricating the same, and relates to an improvement of a buffer layer provided between a substrate and a semiconductor device layer structure, a semiconductor device having a p-type semiconductor layer with reduced resistance and a method of fabricating the same.
2. Description of the Related Art
One of nitrogen-containing III-V group compound semiconductors, i.e. GaN, which has a large band gap of 3.4 eV and is of direct transition type, has been regarded as a prospective material of a short-wavelength light emitting device. Since there is no substrate with good lattice matching with this type of material, the material is often grown on a sapphire substrate. However, since the possibility of non-matching between sapphire and GaN is high, i.e. about 15%, the material tends to grow in an insular shape. Furthermore, if the thickness of a GaN layer is increased to enhance the quality thereof, a difference in thermal expansion between a sapphire substrate and GaN or Al.sub.1-x-y Ga.sub.x In.sub.y N (where 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1, hereinafter referred to as AlGaInN-based material) and lattice non-matching result in an increase in dislocation or cracks at the time of growing and/or cooling. Consequently, it is difficult to grow a high-quality film.
On the other hand, in order to reduce the influence of lattice non-matching, there is a known method in which a very thin film of amorphous or polycrystal AlN or GaN is formed as a buffer layer on a sapphire substrate by low-temperature growth. In this case, it is considered that the amorphous or polycrystal buffer layer reduces thermal distortion, small crystals contained in the buffer layer become orientated seeds at high temperature of 1000.degree. C., and the crystal quality of the GaN layer is enhanced.
In the case of adopting this method, the crystal quality represented by, e.g. a full width at half maximum of X-ray diffraction depends greatly on the growth conditions of the buffer layer. Specifically, when the thickness of the buffer layer is great, the orientation of the seeds, which become the nuclei of the film formation, are disturbed and the crystal quality deteriorates. On the other hand, although the full width at half maximum decreases as the thickness of the buffer layer decreases, the function of the buffer layer is completely lost by an excessively small thickness of the buffer layer and the surface condition of the crystal deteriorates suddenly. In other words, the growth conditions of the buffer layer are strictly limited and the crystal quality is not satisfactory.
As has been stated above, in the prior art, it is difficult to crystal-grow a high-quality AlGaInN-based thin film on the sapphire substrate. Moreover, even if the amorphous or polycrystal buffer layer is used, the growth conditions of the buffer layer are strictly limited and the crystal quality of the AlGaInN-based thin film formed on the buffer layer is not satisfactory. Therefore, it is difficult to fabricate a semiconductor light emitting device with high luminance and short wavelength by using the AlGaInN-based material.
When this type of semiconductor device is used as a semiconductor laser, etc., it is necessary to provide means for forming a low-resistance p-type layer. In the prior art, it is difficult to grow a low-resistance p-type layer with GaN. Recently, however, the resistance of the GaN layer can be decreased by radiating an electron beam or by heating in a nitrogen gas atmosphere the GaN layer to which Mg is added. It appears that the resistance can be decreased by virtue of dissociation of hydrogen from the crystal.
The inventors of the present invention, however, have discovered that the electrical activation ratio of Mg in the GaN layer is still low, and it is necessary to add Mg at a very high concentration of about 10.sup.19 cm.sup.-3 to about 10.sup.20 cm.sup.-3 in order to obtain a low-resistive p-type layer of 1 .OMEGA.cm or less which is necessary to fabricate a high-performance device such as a semiconductor laser. The addition of Mg at high concentration results in an increase in crystal defects and deterioration in surface flatness. Thus, it is not possible to achieve a high-performance, short-wavelength semiconductor laser, etc.
On the other hand, Jap. Pat. Appln. KOKAI Publication No. 5-183189 proposes a method of a low-resistance p-type GaN layer by forming an AlN layer as a cap layer on a GaN layer to which Mg is added, and then annealing the resultant. In this method, however, the electrical activation of Mg is not fully achieved.
As has been described above, in the prior art, when a semiconductor layer of an AlGaInN-based compound (a nitride-series Group III-V compound semiconductor layer) is grown, it is necessary to add Mg (acceptor impurity) excessively as a dopant in order to low-resistance p-type layer. As a result, crystal defects increase and surface flatness deteriorates.